The present invention relates to a method of making valve guides by thermal spray on a cylindrical substrate.
Valve guides are used in aluminum engines to guide the movement of the stems of valves in an engine cylinder head. Prior art cast iron engines normally do not require valve guides due to the self-lubricating qualities of the graphite constituent in the cast iron base material of the cast iron engine heads and the wear-resistant nature of the cast iron itself. The self-lubricating and wear-resistant capacity of cast iron contribute to long life with minimum lubrication. Valve guides for aluminum engine heads are generally produced from cast iron or by powder metal processes and are installed on a machining line along with the valve seats.
Valve guides must resist wear over the life of a vehicle. If a engine valve guide becomes worn it may adversely affect engine durability, oil consumption and emission performance. Aircraft and diesel engine valve guides may be subjected to extreme use and long service life that increases the potential for wear. In automotive applications, wear of engine valve guides can develop over time. Valve guide wear can lead to reduced engine durability and increased oil consumption and emissions in engines that have been operated typically for more than 100,000 miles.
Powder metal valve guides manufactured with steels having a high molybdenum content offer good wear resistance but are relatively costly.
Prior art methods of making valve guides have failed to provide a cost-effective method of making valve guides that are both durable and self-lubricating, thereby minimizing oil consumption and engine emissions at high mileage.
The above problems and objectives are addressed by applicants"" invention as summarized below.
According to the present invention a method of making an engine valve guide is provided by feeding and rotating a tubular mandrel as it is advanced into a spray booth or enclosure. The mandrel is thermally sprayed with a composite layer consisting of from 0-50% by volume of a self-lubricating polymer in a matrix of wear-resisting metal. The self-lubricating, wear-resistant layer on the mandrel is then sprayed with additional, outer layers of a base metal. Predetermined lengths of the mandrel, wear-resistant layer and additional layers are then cut off. The mandrel is then removed from the wear-resistant layer and additional layers leaving a tubular engine valve guide having an internal wear-resistant layer.
The mandrel may either be uncoiled from a roll of tubing and straightened before being fed into the spray booth or, alternatively, may be provided as straight sections.
According to the method, the wear-resistant, self-lubricating material is a composite consisting of a thermally-stable polymer in a matrix consisting of a wear-resistant metal, metal alloy or metal-metal oxide composite, as occurs naturally when oxidizable metals are thermally sprayed in an air or oxygen-containing atmosphere. The method may also comprise flame spraying a powder polymeric material in conjunction with thermal spraying the wear-resistant material. Application of the polymeric material may be simultaneous with or subsequent to beginning application of the wear-resistant material. The polymeric material may be applied by means other than thermal spray such as wet or dry spray application at ambient temperature. The additional layers of material thermally sprayed on the wear-resistant layer are preferably steel.
According to the method, the step of cutting off the mandrel is performed by a flying cut-off machine. After the lengths of tubing have been cut off, the step of removing the mandrel from the wear-resistant layer may be performed in a machining operation. For example, the machining operation may consist of drilling, broaching, water jet cutting, reaming, or combinations of such machining operations. The material forming the mandrel is selected from the group consisting of aluminum, brass, steel or copper.
A cooling medium is directed through the mandrel as the tubing is fed into the spray booth and while the wear-resistant, lubricant and additional layers are applied to the tubing. The cooling medium may be air, water or another fluid that is capable of cooling the mandrel during the spray forming steps.
These and other advantages of the present invention will be more clearly understood in view of the attached drawings and in light of the following detailed description of the invention.